Turns out that moving this aggregation hits an issue in sql plugin. This plugin isn't modularised, when it loads the MedianAbsoluteDeviationAggregationBuilder in MedianAbsoluteDeviationAgg class it loads the builder class using an unnamed module. This causes problems during the search, it can't lookup MedianAbsoluteDeviationAggregatorSupplier, since the key contains a class that is loaded with a class loader using unnamed module. So in MedianAbsoluteDeviationAggregatorSupplier#innerBuild(...) method the call to getAggregator(...) fails.

Also if the search request would be serialised to the search api then this problem wouldn't exist. The same instance that is created in the sql plugin is used in the search action causing these class loading issues.

I think the right solution would be to modularise the sql plugin. But then its dependencies like ql and mapper-version would need to be modularised. The sql-proto dependency is in particular an issue, since it is a java8 library that can't be modularised if it needs to stay compatible with java8. In anyway this change is too large to be done in this PR.